This invention relates to a process of removing monohydric, dihydric and polyhydric phenols, hydrogen sulfide, free and combined ammonia by extraction and distillation from waste water which is formed during the degasification or gasification of coal and is subsequently subjected to biological purification.
To remove phenol from ammonia-containing waste water (also referred to herein as sewage) formed in coking plants, low-temperature carbonization plants, gas-producing plants and the like it is known to thoroughly mix the sewage with a low-boiling, oxygen-containing, organic solvent, such as butylacetate. After the subsequent stratification, a major portion of the phenols is contained in the solvent. When the process is repeated several times in counterflow operation, the phenol can be recovered in a yield of more that 99.9%. The organic solvent required to extract the phenol can be purified by distillation and re-used.
The dephenolized water may be subjected to a biological after-purification, possibly after a removal of ammonia (Lurgi-Handbuch, 1960, pages 64/65).
It is also known to use two solvent cycles in the dephenolization of industrial sewage. A major portion of phenol is removed in the first cycle and the remainder in the second. The solvent for the first cycle is removed from the laden solvent of the second cycle and the solvent which has been recovered by distillation from the first cycle is fed into the second cycle (German Pat. No. 939,268). This process can only be used, however, with two gas water streams which differ in the distribution of phenol and in rate.
Whereas the known treatments can be used to remove mainly monohydric phenols from industrial sewage, the processes become fairly expensive if the polyhydric phenols are to be extracted too. The above-mentioned processes can be used for a complete removal also of dihydric and other polyhydric phenols but this is highly expensive because solvent is required in more than ten times the quantity needed for monohydric phenols. The sewage from which phenol has been removed still contains larger or smaller quantities of fatty acids, cyanides, and other impurities, some of which can be biologically decomposed only with difficulty. It would not be economical to remove these constituents chemically. On the other hand, sewage must meet strict requirements as to purity, particularly when the sewage is to be used, e.g., as cooling water.
In case of a very high content of polyhydric phenols and fatty acids, the usual extraction of the monohydric phenols, e.g., with 10% isopropylether in ten stages, results in a dephenolized water which still contains a large proportion of these substances so that the loading of the subsequent biological purification is very high. In the latter, the formation and removal of sludge constitute a great problem.